The invention relates to cermets based on transition metal borides, especially titanium diboride, which have an improved toughness.
Cermets are hard materials particularly useful for making metal cutting and rock drilling tools, as well as wear parts. As their name indicates they in general, contain a hard but brittle ceramic phase as a major constituent, and a metallic binder phase less hard but tough, which gives them an interesting and rare combination of hardness and toughness. The notation ceramic is taken in a broad sense, including in particular the oxides, nitrides, carbides and borides of the transition metals, as well as their combinations.
It is known that certain metals such as iron, nickel, cobalt, chromium, copper, etc., and their alloys, have been used as binders in the fabrication of cermets based on transition metal borides, most commonly diborides of these metals, and particularly titanium diboride TiB.sub.2. These metals or alloys have, in principle, two functions. First, they ensure the formation of a liquid phase, often with a certain quantity of the boride dissolved in this liquid phase, thus wetting as well as possible the solid component, which, in principle, facilitates the sintering and permits a complete densification. Furthermore, they contribute a certain toughness to the sintered cermet which thus includes a hard but brittle phase (the boride) and a metallic binder, less hard, but ductile (the binder metal or alloy).
In reality, however, the study of the literature shows that experimentally the densification by liquid phase sintering of such cermets is not perfect and often insufficient. The open porosity remains significant (4 to 30 volume %), and the metallic binder is often transformed, at least partially, to boride through a chemical reaction with the hard phase. This results in a considerable decrease in the toughness of the cermet, which restricts possible fields of application.
The inventors have confirmed these findings experimentally, particularly for the cermets TiB.sub.2 --Fe. In order to make such cermets, powder of titanium boride TiB.sub.2 (average grain diameter: 1 to some/.mu.m) was mixed with iron powder (average grain diameter: 1 to some .mu.m) using conventional means such as mixer, ball mill, attritor mill, etc. The mixture was subsequently compacted under a pressure of 100 to 200 MPa. The compacted product was sintered for 1 to 4 hours, at a temperature of between 1450.degree. and 1550.degree. C., depending on the amount of iron in the cermet (10 to 20 volume %). It was found that the densification was very poor (the remaining porosity varying between 10 and 20%) and that the major part of the binder iron metal had been transformed to the brittle Fe.sub.2 B and/or FeB boride, thus causing a decrease in toughness. This makes the use of such a material practically impossible for the envisaged applications.
In it appeared practically unrealistic to obtain a boride metal (or alloy) cermet due to the interaction, during sintering, of the metallic binder and the hard boride, with the at least partial boruration of the metallic binder.
Accordingly, an object of the present invention is to find the conditions which permit the fabrication of cermets based on transition metal borides without presenting the disadvantages of cermets of this type made according to prior art, and thus permitting their use in the envisaged applications because of their high toughness.